7950gt gx2 question??

is the 7950gt or gx2 1 physical card or 2 physical cards that make up 1 card or

is 2 cards that sli?

then does it work with only 1 of the physical cards if its 2 physical card that

makes up 1 card?

and

with the 7950gt or gx2 in sli mode do you need 4 pci e slots??